Conventional articles of athletic footwear include two primary elements, an upper and a sole assembly. The upper provides a covering for the foot that comfortably receives and securely positions the foot with respect to the sole assembly. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole assembly is secured to a lower portion of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces, the sole assembly may provide traction, control foot motions (e.g., by resisting over pronation), and impart stability, for example. Accordingly, the upper and the sole assembly operate cooperatively to provide a comfortable structure that is suited for a wide variety of activities, such as walking and running. An insole may be located within the upper and adjacent to a plantar (i.e., lower) surface of the foot to enhance footwear comfort, and is typically a thin, compressible member.
The sole assembly may incorporate multiple layers. Some footwear include only a midsole, while others may also include an outsole secured to a bottom surface of the midsole. The midsole, which is conventionally secured to the upper along the length of the upper, is primarily responsible for attenuating ground reaction forces. The midsole may also form the ground-contacting element of footwear. In such embodiments, the midsole may include texturing, such as projections and recesses or grooves, in order to improve traction. The outsole, when present, forms the ground-contacting element and may be fashioned from a durable, wear-resistant material.
The midsole may be primarily formed from a resilient, polymer foam material, such as ethylvinylacetate (EVA), that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness and degree of ground reaction force attenuation may be altered to meet the specific demands of the activity for which the footwear is intended to be used. In addition to polymer foam materials, conventional midsoles may include, for example, one or more fluid-filled bladders and moderators.
The sole assembly may be formed of multiple portions, with some or all of the portions having different colors. When EVA is formed in a mold assembly the color lines between the different colored portions may bleed, decreasing the aesthetic appeal of the footwear.
It would be desirable to provide a midsole formed from two preforms and a method and mold for manufacturing such a midsole that reduces or overcomes some or all of the difficulties inherent in prior known devices. Particular advantages will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain embodiments.